Insulated solid and stranded electrical cables are well known in the art. Generally stranded cables include a central stranded conductor with a protecting insulation jacket disposed around the conductor.
The most frequent cause of failure of directly buried aluminum secondary cables is a cut or puncture in the insulation inflicted during or after installation. This leads to alternating current corrosion of the aluminum and finally to an open circuit. When a conductor is exposed to wet soil, upon damage, leakage current may flow, and cause localized electrochemical conversion of aluminum to hydrated aluminum oxide and eventually to an open circuit of the conductor.
In the U.S., thousands of such instances occur annually and the repair (location, excavation, repair, and replacement) can be very costly. As a result of the failures and in response to this problem, a tougher insulation system was introduced and became an industry standard. The tougher cable is described as “ruggedized,” and generally consists of two layers: an inner layer of low density weight polyethylene and an outer layer of high density polyethylene. This design is more resistant to mechanical damage than one pass low density polyethylene, but still can result in exposure of the aluminum conductor if sufficient impact is involved.
Investigations show that AC electrolysis current can approach half-wave rectification when the current density is high. This accounts for the rapid loss of aluminum metal frequently experienced in the field. A caustic solution (pH 10-12) develops at the aluminum surface and dissolves the protective oxide film.
The mechanism of aluminum cable failure is the formation of hydrous aluminum oxide. As the aluminum oxide solids build up, the insulation in the vicinity of the puncture is forced to swell and splits open, making larger areas of the aluminum conductor surface available for electrolysis, thus increasing the leakage current and accelerating the corrosion process. Rapid loss of aluminum by AC electrolysis continues until ultimately the cable is open-circuited. A caustic environment is created at the aluminum, electrolyte interface, which dissolves the protective oxide film.
The ruggedized or abuse resistant type insulation was supposed to protect the cable from physical abuse. While it helped this problem, it did not eliminate 600 V cable failures. Utilities have recently reported varying numbers of 600 V aluminum underground distribution cable failure rates scattered between 70 and 7000 per year. Failures are evidenced by an open circuit condition accompanied by severe corrosion of the aluminum conductor.
All the reasons for 600 V failures are unknown, but several have been postulated by cable users. These cables seem to experience a high degree of infant mortality, followed by failures occurring over decades. The infant mortalities are usually directly related to damage caused by adjacent utilities, damage inflicted by landscaping and planting, or damage to the cable prior to or during installation. The failures occurring years later are harder to explain. There have been postulations of lightning damage, manufacturing defects, or insulation degradation over the life of the installation.
In order to better understand the insulation characteristics, studies of the AC breakdown, and DC impulse breakdown were conducted. AC breakdown studies on several different cables showed a high safety margin of performance. Each of these cables had a 0.080 inch wall thickness. Tests were conducted in water filled conduits. The AC breakdown strength of all of these cables was consistently above 20 kV, far in excess of the operating stress.
Impulse breakdown studies have also been performed on several 600 V cable constructions having different insulation formulations. The impulse breakdown level of these cables was approximately 150 kV. This exceeds the BIL requirements of a 15 kV cable system and should well exceed the impulses on 600 V secondary cables during operation.
The above margins of electrical performance were measured on new cables. They are far above what is needed to operate on a 600 V system since most of these cables operate at 120 V to ground. One of the tests during compound and product development is a long term insulation resistance test performed in water at the rated operating temperature of the insulation. For crosslinked polyethylene cables the water temperature is 90° C. The insulation resistance must demonstrate stability and be above minimum values for a minimum of twelve weeks. If there is instability indicated, the test is continued indefinitely. Relative permittivity is measured at 80 v/mil and must meet specific values. Increase in capacitance and dissipation factor are also measured in 90° C. water over a 14 day period. Insulation compounds used in present day cables easily meet these requirements.
Manufacturing defects in cable insulation are found during production by either of two methods. During the extrusion process, the cable is sent through a spark tester, where 28 kV DC, or 17 kV AC, is applied to the insulation surface. Any manufacturing defect resulting in a hole in the insulation will initiate a discharge, which is detected by the spark tester. Most manufacturers use this method. Another test that is also often employed is a full reel water immersion test. In this test 21 kV DC, or 7 kV AC is applied to the cable after immersion for 1 hour or 6 hours, depending on whether the cable is a plexed assembly or single conductor, respectively. The actual voltages used for these tests are dependent on the wall thickness. The above values are for an 0.080 inch wall.
The above testing has demonstrated electrical performance that is stable and far surpasses the requirements of the installation for 600 V cable. This does not explain a sudden cable failure after many years of operation. Such sudden failure can be explained by a better understanding of the failure mechanism. Aluminum corrosion in the presence of an alternating leakage current is a combination of two different mechanisms. Aluminum is normally afforded a great deal of corrosion protection by a relatively thin barrier layer of aluminum oxide, and a more permeable bulk layer of oxide. However, flaws or cracks exist in these layers which provides a spot for the corrosion reaction to begin. The metal in contact with water undergoes an anodic (positive ions moving into solution) and a cathodic cycle, sixty times per second.
During the anodic half cycle of leakage current, aluminum ions leave the metallic surface through these flaws and combine with hydroxyl ions in the water surrounding the cable. This reaction results in pitting of the metal and the formation of aluminum hydroxide, the whitish powder evident in corroded cables. Another important reaction also occurs. The hydroxyl ions are attracted to the metal surface during this half cycle, which increases the pH, causing a caustic deterioration of the oxide layer, further exposing more aluminum.
During the cathodic half cycle another reaction occurs. Hydrogen ions are driven to the aluminum surface. Instead of neutralizing the caustic hydroxyl concentration, the hydrogen ions combine and form hydrogen gas, which leaves the cable. The hydrogen depletion has the effect of further concentrating the caustic hydroxyl ions, thus furthering the deterioration of the surface oxide. No pitting occurs during this half cycle since the aluminum ion is attracted to the metal. A caustic solution develops, hydrogen evolves, aluminum pitting takes place, and aluminum hydroxide forms during this reaction.
A critical current density is necessary to sustain the corrosion reaction. Below this current density corrosion will be very slight, or almost imperceptible. Once the current density is high enough, the reaction can be swift. The necessary current density is below 1 mA/in2. The current density of a damaged 600 V cable is influenced by the voltage, leakage resistance, and the area of exposed metal. Variables affecting this can include dampness of the soil, chemistry of the soil, degree of damage, etc.